Sucrose esters/cosurfactant microemulsion systems for transdermal delivery: Assessment of bicontinuous structures

This research studies microemulsion systems containing minimal amounts of surfactant for topical use. Sucrose esters are not able to form microemulsions without a cosurfactant. Microemulsion areas were investigated for numerous systems including sucrose esters/cetearyl octanoate/alcohols/water at different surfactant/cosurfactant mass ratios, called K_m, and different HLB values. The pseudoternary isotherm diagrams were constructed by titration at 25°C. The long and unsaturated oleic chain of the surfactant improves the extent of the microemulsion zone and when K_m increases the domain becomes larger. A combination of laurates at HLB = 7 increases the water and oil solubilizing capacity. Cosurfactants affect the shape and the extent of microemulsion regions. Shorter alcohols which are expected to disorder the interfacial film gave extended microemulsion zones by destabilizing the liquid crystalline phases. Moreover, in short alcohol based diagrams, the microemulsion areas are single volume in which the percolation transition law can be applied. The electroconductive behaviour allows us to determine the percolation threshold and to identify quantitatively bicontinuous structures. These structures, owing to their very low interfacial tension, associated to their wetting properties, should be very interesting as new drug carrier systems for transdermal delivery.